This invention relates to an apparatus for use in electron microscopes which improves the image resolution of light images formed therein.
Transmission electron microscopes use a beam of accelerated electrons which pass through a sample to provide an electron image and/or diffraction pattern of the sample. To provide a record of these images and/or diffraction patterns, the electrons have been converted into light images using scintillator materials (e.g., phosphors), and the light images and/or patterns are then captured by a camera. While photographic film has long been used, charge-coupled devices (CCD) of the type-originally developed for astronomy to read light images into a computer have found increasing use in this field. Such CCD cameras offer excellent resolution, sensitivity, linearity, up to 2,048.times.2,048 pixels, are reusable, and make the image available for viewing within seconds of recording.
The final resolution of any camera recording these images and patterns is determined by the combined effect of 1) scattering of the incident electrons by atoms in the scintillator material and supporting structure for the scintillator, 2) spreading and random scattering of the electron generated photons by boundary and grain surfaces in the scintillator, and 3) the resolution of the transfer optics from the scintillator to the camera. The electron scattering is dependent on the accelerating voltage of the imaging electrons and becomes the limiting factor in ultimate resolution for electrons accelerated to 200 keV or higher. At lower voltages, electron scattering is still a factor in determining resolution, and must still be minimized to achieve the best possible resolution.
Certain applications such as tomography require the use of very high accelerating voltages. Because of the thicker samples used in tomography and the rotation used to achieve a three-dimensional image, higher accelerating voltages must be used for transmission of electrons through such samples. Accelerating voltages of 1 MeV and higher are often used.
While resolution loss due to electron scattering can be reduced by making the scintillator material very thin, this necessitates mounting the scintillator on a support structure. When a fiber optic plate is used as the transfer optic (from the scintillator to the camera), it can be used to form a natural support structure for the scintillator. When lens optics are used as the transfer optic, the scintillator typically may be mounted on a transparent piece of glass.
The incident electrons deviate randomly from their initial direction as they move through the solid material of the scintillator and support structure. After the electrons pass through the scintillator, no more light is produced. However, some fraction of the electrons are deflected enough as they pass through the support structure that they are directed back into the scintillator where they cause more light to be emitted, this time at some greater distance from their initial point of entry. Thus, resolution is degraded by scattering in the support structure even though it is not scintillating.
At primary energies of greater than about 0.6 MeV, electrons that have randomly deviated from their initial path, spread sideways in the support structure, and have then been scattered back to the scintillator material produce an unacceptable "tail" in the point spread function (PSF) of the camera. At 0.8 MeV and above, there is also greatly increased damage to the support structure. Such damage to the fiber optics or glass support may be due to an accumulation of high energy electrons in the support leading to volume discharges.
Of additional concern is that with high voltage electrons, there are also forward-going X-rays which are generated by the electrons. Such X-rays are able to penetrate through several centimeters of support structure, such as fiber optics. This may result in an unacceptably high spurious X-ray background in the images. Alternatively, if the thickness of the fiber optics is increased to reduce the X-ray background, then the light intensity of the images will be reduced by factors as high as 10:1.
Accordingly, the need still exists for an apparatus which will improve the ultimate resolution of cameras used to record images from electron microscopes.